This invention relates to a semiconductor device having a semiconductor body comprising at least two complementary transistors and consisting of a substrate of a first conductivity type which is covered with a first epitaxial layer of the second conductivity type opposite to the first, at least a part of the substrate constituting the collector zone of the first transistor; the base zone of said first transistor and the collector zone of the second transistor being constituted by adjacent parts of the said first epitaxial layer.
In numerous power amplifier circuits the output transistor is controlled by a transistor of the complementary type, for example, an npn-transistor by a pnp-transistor. A pnp-transistor may also be used for controlling an assembly of two npn transistors which is known as a Darlington amplifier. With a view to the integration of such circuits in one single device, it has been endeavoured to manufacture in the same part of the semiconductor plate at least one npn-transistor and one pnp-transistor, in which, for example, the base zone of the first transistor is connected to the collector zone of the second transistor.
It is known that there are substituted advantages associated with a structure having two integrated complementary transistors that both have an epitaxial base zone and collector zone. Known devices, in which one of the transistors has a diffused base zone actually have as drawbacks, for example, lower definition of the thickness of the base zone, an irregular doping concentration and difficulties in controlling the characteristics. The two transistors of these devices are different and the variation of the characteristics with temperature disturbs the operation considerably. The output transistor, which usually is a power transistor, is difficult to manufacture by diffusion operations. In addition, planar transistors having a diffused base are relatively more sensitive to the occurrence of second breakdown than are transistors having an epitaxial base.
In French patent application No. 7,303,748 published under No. 2,216,678 and filed on Feb. 2, 1973, applicants describe a structure of which the two transistors have an epitaxial base zone and collector zone which yields a great number of advantages. However, this structure also suffers from drawbacks, both in itself, and in the methods which are necessary to obtain said structure.
In the preferred embodiment of applicants' earlier structure (in which the two transistors adjoin the same surface) an epitaxial layer of a first conductivity type has to be etched away, and epitaxial layer of the opposite conductivity type has then to be grown in the resulting aperture and constitutes the base zone of one of the two transistors, in which base zone the emitter zone of the same transistor is then diffused. Hence it is difficult, on the one hand, to obtain by etching a window of a uniform depth and a correct surface state (whence a poor definition of the thickness of the base zone and nonuniformities in the basecollector junction), and on the other hand it is also difficult to form a local epitaxial layer.
In the second embodiment of the applicants' earlier structure (in which one of the transistors is at an elevation relative to the other one) a part of an epitaxial layer has also to be removed so as to reach a deeper layer and to manufacture in the surface of said deeper layer the emitter zone of one of the two transistors. Since the etching runs off irregularly, the deep layer is more or less subject to initial etching and the surface thereof is very nonuniform; consequently, the emitter zone which is diffused at this surface has a poorly defined depth and the underlying emitterbase junction has a capricious profile. As regards the structure itself, it is known in addition that when the profile of the active surface of a semiconductor structure shows unevennesses, drawbacks result therefrom, as for example, the difficulty of performing accurate photo-etching treatments and in particular a possibility of cracking of the insulating and conducting layers which are provided on the unevennesses.